The present invention is concerned with methods and apparatus for detecting the presence of an intruder within a preselected area. More particularly, the present invention relates to methods and apparatus for sensing the presence of ambient field modulation by movement of an intruder through antenna detection of the wave patterns present within an area to be protected. The present invention is particularly useful in conjunction with security monitoring of rooms, buildings, or the like and is equally applicable to utilization within residences, business establishments, open areas having substantial utility power line originated ambient electromagnetic fields present and the like.
It has been known for some time that movement of a person within an area wherein there is ambient electromagnetic signals produces a low frequency modulation of those signals in the range of from 0 to 20 hz. For instance, U.S. Pat. No. 3,163,861 by Suter uses the reflected radio frequency signals from a commercial frequency-modulation transmitter to detect intruder presence in a protected area. A broad band AM detector monitors the FM envelope and sets off an alarm if this envelope varies from a balanced condition. Suter selects commercial FM stations transmissions for this purpose since the wave length is roughly proportional to the size of the average person and thus most likely to provide stronger reflective modulation signals. A similar approach is employed by U.S. Pat. Nos. 3,237,105 by Kalmus and 3,378,834 by Corbell except that discrete, controlled transmitters are employed rather than a commercial FM transmission. Kalmus and Corbell employ carefully located transmitter and receiving antennas so as to detect movement therebetween by the intended intrusion detector. All such prior art devices are severely limited in that careful location of the transmitter and receiving antennas must be maintained and further are generally incapable of significantly differentiating over transient signals not caused by an intruder. Thus, signals such as telephone ringing, pulses, refrigerator motor actuations and the like are sensed by such systems and produce alarm signals falsely indicating the presence of an intruder. Similar limitations are present for U.S. Pat. No. 3,833,897 by Bell et al which uses integrator circuits in an effort to distinguish signals originating from disturbances of an "electret" cable on a fence or the like.
Other prior art devices have used light sensitive circuits to detect intervention by an intruder such as U.S. Pat. No. 3,549,892 by Perlman and U.S. Pat. No. 3,727,207 by Missio et al. Perlman cyclically gates the output of a light sensitive receiver so that an integrator must receive a sufficiently strong input to rapidly accumulate an output beyond a preselected threshold level during a time period established by a monostable multivibrator before a alarm is generated. Missio et al uses logic circuitry to detect that reflected pulses occur out of tolerance as established by flip-flop circuits.
Yet other intrusion detector systems are contingent upon capacitive variation sensing or static charge detection. Unfortunately, such systems are sensitive to uncontrollable variables such as humidity variations and the like which render them of little value in many practical applications.
Accordingly, there has been a continuing need for an intrusion detector system which can operate reliably in ambient conditions without requiring special tailoring of transmitter and receiver antennas along with the attendant requirements for proper location thereof. Further, there has been a continuing need for an intrusion detector system which can be easily adapted to the areas requiring surveillance while allowing manufacture with minimal cost so as to allow utilization in a large variety of environments. Ideally, such a system should be easily installable and require minimal attention and adjustment after installation despite varying conditions within the environment. The system should allow the user to select sensitivity so as to differentiate between different objects within the protected area.